Natural gas has been used to fuel vehicle engines for many years. The fuel supplied to a natural gas driven vehicle is stored either in a liquefied natural gas (LNG) tank or in a compressed natural gas (CNG) cylinder.
CNG is normally stored at ambient temperatures at pressures up to 3600 pounds per square inch while LNG is normally stored in a cryogenic storage vessel at temperatures between about −240° F. and −175° F. (about −150° C. and −115° C.) and at pressures between 15 and 200 psig. CNG has been the more broadly adopted form of fuel storage but it has a lower energy density compared to LNG. Now that natural gas is gaining greater acceptance as a fuel for transportation, the benefit of the higher energy density for LNG is attractive for vehicles that require a longer range between re-fuelling. LNG provides an energy density of about four times that of CNG with the aforementioned storage conditions. Increased demand for natural gas engines has increased the need to develop improved on-board fuel supply systems with natural gas stored as LNG instead of CNG.
Natural gas fuelled engines can operate by injecting the fuel in the engine's air intake manifold or by injecting the fuel directly into the engine's combustion chambers. In systems where fuel is injected into the engine's air intake system the required fuel supply pressure is relatively low, for example around 100 psig. In systems where the fuel is injected directly into the engine's combustion chamber and where the fuel injection pressure needs to be higher than the in-cylinder pressure the fuel supply system has to deliver natural gas to the injector at higher pressures, for example at pressures of at least 3000 psig.
Current LNG low pressure fuelling systems generally consist of a liquid conduit which supplies liquid fuel from the storage vessel to the engine through a supply line comprising a vaporizer. Fuel is stored in the storage vessel at a pressure of around 150 psi which is higher than the fuel delivery pressure to the engine which can be between 70 to 100 psi.
In these systems, heat is transferred to the LNG fuel storage vessel and a portion of the stored liquid fuel can vaporize thereby increasing the pressure inside the vessel. One method of relieving the pressure inside an LNG storage vessel is to vent the gas into the atmosphere. However this method is wasteful and can be represent a hazard.
As an alternative, the existing delivery systems for low pressure engines are provided with an economizer circuit which consists of a vapor conduit through which vapor can be withdrawn from the vapor headspace in the storage vessel and can be supplied to the engine. In engine supply systems provided with an economizer circuit, most of the time, the engine operates with LNG fuel supplied from the liquid space in the storage vessel and vapor is pulled from the vapor headspace only when the pressure in the storage vessel rises above a predetermined limit set by a regulator placed in the vapour conduit. An example of such a system is described in U.S. Pat. No. 5,421,161 which illustrates an economizer circuit including means for providing a fixed back pressure in the liquid withdrawal conduit such that when the pressure in the storage vessel increases over a predetermined amount, the path of least resistance is through the vapor conduit and vapor is preferentially withdrawn from the vapor headspace of the storage vessel to thereby lower the pressure within the LNG storage tank faster.
The known systems using LNG storage vessels to supply fuel to a low pressure internal combustion engine fuelled with natural gas rely on a high saturation pressure in the storage vessel to push fuel out of the liquid space of the storage vessel and to the engine. When heat is transferred from the surrounding environment to the storage vessel, the saturation pressure of the LNG increases and can be sufficient for pushing fuel out of the vessel. When the engine operates at high load the saturation pressure of the LNG in the storage vessel can drop below a level that is required for pushing fuel out of the tank. As a result the engine becomes starved of fuel and can run in underperforming conditions. In these situations, the driver has to stop the vehicle and wait for the pressure in the storage vessel to increase until he can restart the vehicle.
One known method of increasing the pressure in the LNG storage vessel is to use pressure building coils interposed between the walls of a double-walled cryogenic tank which circulate low temperature fuel from the tank. Heat transferred from to the exterior through the wall of the LNG storage vessel to the pressure building coil vaporizes the liquid fuel and the created vapor can be supplied directly into the headspace of the storage vessel through a regulator when the pressure in the headspace becomes lower than a predetermined value. Such a pressure building circuit is described in U.S. Pat. No. 4,947,651. In other pressure building circuits used for cryogenic tanks in general, such as the one described in U.S. Pat. No. 5,937,655, the pressure building coil is external to the cryogenic tank. In such systems cryogenic liquid from the tank is fed to a pressure builder heat exchanger where the liquid is vaporized and the produced gas is delivered to the tank to pressurize it.
One disadvantage of using pressure building circuits to pressurize a cryogenic fluid storage vessel is that the heat used for generating the vapor that is supplied to the headspace of the storage vessel to pressurize it, is also transferred to the liquid contained in the vessel reducing the vessel's fluid holding time and requiring more frequent venting.
Another disadvantage of the existing low pressure fuel supply systems which rely on the fuel saturation pressure in the storage vessel to supply fuel to the engine is that they cannot adequately supply engines with fuel under transient conditions, when the required fuel supply pressure can vary dynamically between a lower pressure that is required for low load operation and a higher pressure that is required for high load operation. This issue becomes important when natural gas fuel systems are installed on larger vehicles, such as heavy duty trucks, which have larger engines that consume fuel at a higher rate.
Accordingly, there is a need for a method of reliably delivering fuel from a cryogenic storage vessel into the air intake system of a gaseous fuelled internal combustion engine at low pressures, so that fuel is delivered to the engine at the required fuel injection pressure at most, if not all, times and during different engine operating modes including during transients.